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The main difference is the accuracy:
Crystals have a small tolerance (30 to 50 ppm) and resonators around 1% of the nominal frequency.
Resonators usually do not need capacitors to work in a oscillator (the three terminal package, the two terminal package needs).
If I am not wrong, crystals have less thermal deviation also.
Ceramic resonators are made of high-stability piezoelectric ceramics, generally lead zirconium titanate (PZT) which functions as a mechanical resonator. When voltage is applied, its piezoelectric "vibration behavior" causes an oscillating signal. The thickness of the ceramic substrate determines the resonance frequency of the device.
A ceramic resonator is often used in place of quartz crystals as a reference clock or signal generator in electronic circuitry because of its lower cost and smaller size. It is used on circuits where frequency specifications aren't highly critical (quartz has a 0.001% frequency tolerance, while PZT has a 0.5% tolerance).